This invention relates to projection lanterns and, more particularly, to a new and improved catadioptric projection lantern for projecting images of transparencies on a projection screen.
Modern office copying machines are capable of producing full-size (e.g., 8.5 inch by 11.0 inch) transparencies which may then be used in an "overhead projector" to project information onto a screen during a conference or in similar situations. In such overhead projectors, the transparency is placed on an illuminated platen, which is usually disposed in a horizontal orientation, and light transmitted by the transparency is reflected through an overhead mirror to a projection lens, which in turn reproduces an enlarged image of the transparency on a projection screen. The platen upon which the transparency is placed may be lighted from beneath by a light source and condensing lens system which is often bulky and heavy in view of the large platen area to be illuminated. Alternatively, to reduce the overall bulk and weight of the projection lantern, the light source may be in the overhead portion of the projector, and a reflecting Fresnel lens is mounted in the platen on which the transparency is placed.
When transparencies are made with certain types of ink, such as "hot melt" inks used in thermal transfer printers and certain ink jet printers, the inks do not penetrate into the transparency substrate but adhere to the surface of the substrate and retain a three-dimensional form. Thus, hot melt inks which are solid at room temperature and are liquefied by heating for application and then resolidified by freezing on the substrate are distinct from solvent-based inks which dry by evaporation or absorption and leave only a microscopically thin residue on the surface of the substrate. When such three-dimensional ink spots are formed on a transparency substrate, they refract the light transmitted through them in the manner of a dioptric lenticule. As a result, transparencies made with such inks disperse light passing through the ink spots so that most of it is not collected by the projection lens, producing gray shadows rather than colored images when projected by conventional projection lanterns.